1. Field of the Invention
The invention relates to a method and a device for determining estimated values for transmission symbols within a MIMO-OFDM system.
2. Discussion of the Background
In mobile transmission technology with high transmission rates and with low-to-medium mobility of the terminal equipment, multiple antenna systems (Multiple-Input-Multiple-Output (MIMO)-systems) with multiple access according to the orthogonal-frequency multiplex (Orthogonal-Frequency-Division-Multiplexing (FDM)) are increasingly used. The mobile WIMAX standard IEEE802.16e represents an important standard for MIMO-OFDM systems of this kind.
Regarding the technical background, reference is made to US 2008/0063103 A1, which discloses a method for the correction of Alamonti coded MIMO-OFDM signals.
In particular with transmission systems with four transmission antennas, the “Matrix B” transmission mode has proved successful for test purposes, because only two reception antennas are required. Distribution or coding of the individual data symbols s1,s2,s3,s4, . . . to be transmitted to the four transmission antennas m=1,2,3,4 at each of four successive OFDM symbol timing points l,l+1,l+2,l+3 according to the “Matrix B” transmission mode is illustrated in the table in FIG. 1. As shown in the table in FIG. 2, in order to achieve an additional channel coding (“sub-carrier randomising”) in the “Matrix B” transmission mode, a weighting of the data symbols s1,s2,s3,s4, . . . to be transmitted, on each of the four transmission antennas m=1,2,3,4, is implemented additionally, at each of four successive OFDM symbol timing points l,l+1,l+2,l+3 with coefficients v1,v2,v3,v4 which can each adopt the values +1 or −1.
In order to estimate the transmitted data symbols within the framework of the decoding of the reception symbols, on the one hand, the coefficients v1,v2,v3,v4 and, on the other hand, the channel-transmission factors of the transmission channels between each transmission antenna and each reception antenna, must be estimated. The estimation of the channel transmission factors is typically implemented on the basis of pilot symbols. For this purpose, either standardized pilot symbols known to the receiver or arbitrary pilot symbols not known to the receiver are used. With a use of arbitrary pilot symbols, the individual channel transmission factors can in fact be correctly estimated in their modulus using currently available “Matrix B” decoding methods. However, with currently available “Matrix B” decoding methods, the individual transmission factors provide an uncertainty of 180° in their phase. Since the individual channel transmission factors and also the individual coefficients for channel coding cannot be accurately determined with regard to their sign using the currently available decoding method, the transmitted data symbols also cannot be determined accurately.